/*
 * Copyright 2011 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package com.google.gwt.user.client.ui;

import com.google.gwt.core.client.GWT;
import com.google.gwt.core.client.JavaScriptObject;
import com.google.gwt.dom.client.Document;
import com.google.gwt.dom.client.Element;

/**
 * Implementation of scrolling behavior.
 */
class ScrollImpl {

  /**
   * IE does not fire a scroll event when the scrollable element or the
   * container is resized, so we synthesize one as needed. IE scrolls in the
   * positive direction, even in RTL mode.
   */
  static class ScrollImplTrident extends ScrollImpl {

    private static JavaScriptObject scrollHandler;

    private static JavaScriptObject resizeHandler;

    /**
     * Creates static, leak-safe scroll/resize handlers.
     */
    private static native void initStaticHandlers() /*-{
      // caches last scroll position
      @com.google.gwt.user.client.ui.ScrollImpl.ScrollImplTrident::scrollHandler = function() {
        var scrollableElem = $wnd.event.srcElement;
        scrollableElem.__lastScrollTop = scrollableElem.scrollTop;
        scrollableElem.__lastScrollLeft = scrollableElem.scrollLeft;
      };
      // watches for resizes that should fire a fake scroll event
      @com.google.gwt.user.client.ui.ScrollImpl.ScrollImplTrident::resizeHandler = function() {
        var scrollableElem = $wnd.event.srcElement;
        if (scrollableElem.__isScrollContainer) {
          scrollableElem = scrollableElem.parentNode;
        }
        // Give the browser a chance to fire a native scroll event before synthesizing one.
        setTimeout($entry(function() {
          // Trigger a synthetic scroll event if the scroll position changes.
          if (scrollableElem.scrollTop != scrollableElem.__lastScrollTop ||
              scrollableElem.scrollLeft != scrollableElem.__lastScrollLeft) {
            // Update scroll positions.
            scrollableElem.__lastScrollTop = scrollableElem.scrollTop;
            scrollableElem.__lastScrollLeft = scrollableElem.scrollLeft;
            @com.google.gwt.user.client.ui.ScrollImpl.ScrollImplTrident::triggerScrollEvent(Lcom/google/gwt/dom/client/Element;)
              (scrollableElem);
          }
        }), 1);
      };
    }-*/;

    private static void triggerScrollEvent(Element elem) {
      elem.dispatchEvent(Document.get().createScrollEvent());
    }

    ScrollImplTrident() {
      initStaticHandlers();
    }

    @Override
    public native void initialize(Element scrollable, Element container) /*-{
      // Remember the last scroll position.
      scrollable.__lastScrollTop = scrollable.__lastScrollLeft = 0;
      scrollable.attachEvent('onscroll',
        @com.google.gwt.user.client.ui.ScrollImpl.ScrollImplTrident::scrollHandler);

      // Detect if the scrollable element or the container within it changes
      // size, either of which could affect the scroll position.
      scrollable.attachEvent('onresize',
        @com.google.gwt.user.client.ui.ScrollImpl.ScrollImplTrident::resizeHandler);
      container.attachEvent('onresize',
        @com.google.gwt.user.client.ui.ScrollImpl.ScrollImplTrident::resizeHandler);
      // use boolean (primitive, won't leak) hint to resizeHandler that its the container
      container.__isScrollContainer = true;
    }-*/;

    @Override
    public native boolean isRtl(Element scrollable) /*-{
      return scrollable.currentStyle.direction == 'rtl';
    }-*/;
  }

  private static ScrollImpl impl;

  /**
   * Get the singleton instance of {@link ScrollImpl}.
   */
  static ScrollImpl get() {
    if (impl == null) {
      impl = GWT.create(ScrollImpl.class);
    }
    return impl;
  }

  /**
   * Get the maximum horizontal scroll position.
   * 
   * @param scrollable the scrollable element
   * @return the maximum scroll position
   */
  public int getMaximumHorizontalScrollPosition(Element scrollable) {
    return isRtl(scrollable) ? 0 : scrollable.getScrollWidth() - scrollable.getClientWidth();
  }

  /**
   * Get the minimum horizontal scroll position.
   * 
   * @param scrollable the scrollable element
   * @return the minimum scroll position
   */
  public int getMinimumHorizontalScrollPosition(Element scrollable) {
    return isRtl(scrollable) ? scrollable.getClientWidth() - scrollable.getScrollWidth() : 0;
  }

  /**
   * Initialize a scrollable element.
   * 
   * @param scrollable the scrollable element
   * @param container the container
   */
  public void initialize(Element scrollable, Element container) {
    // Overridden by ScrollImplTrident.
  }

  /**
   * Check if the specified element has an RTL direction. We can't base this on
   * the current locale because the user can modify the direction at the DOM
   * level.
   * 
   * @param scrollable the scrollable element
   * @return true if the direction is RTL, false if LTR
   */
  public native boolean isRtl(Element scrollable) /*-{
    var computedStyle = $doc.defaultView.getComputedStyle(scrollable, null);
    return computedStyle.getPropertyValue('direction') == 'rtl';
  }-*/;
}